This invention relates to sensors for determining the position of an object, in particular determining the object's lateral displacement and rotational orientation, using capacitance.
Positional sensors have various applications in today's world. In some applications, a very accurate measurement of position is needed over a limited range. One example of such an application is in manipulation of a stage of a high resolution microscope such as an electron microscope. Electron microscopes include Scanning Electron Microscopes (SEMs), Transmission Electron Microscopes (TEMs), Scanning Transmission Electron Microscopes (STEMs) and various kinds of Reflection and Emission Electron Microscopes like the Low Energy Electron Microscope (LEEM) and Photoemission electron Microscope (PEEM). Such microscopes have much higher resolution than optical microscopes and therefore require manipulation of a stage with very high precision. The location of the specimen stage is generally measured as it is moved so that when a feature is found, its location may be recorded and the microscope may return to the feature if desired. In addition to measuring the location of the stage, the angle of tilt of the stage with respect to the electron beam illumination has to be measured to align the objective lens properly. Thus, the position of the stage may include location in three translational and two angular degrees of freedom of the stage.
Optical encoders may be used to measure the position of an electron microscope stage. Even though modern optical encoders overcome the light wavelength limit by interpolation and can measure down to a few nm displacement, the direct measurement of displacement is only possible within the periodicity of the ruler used, typically a few um. When the displacement is larger, a counter for the traversed ruler periods is necessary. The counter can get out of synchronization with the ruler, which usually means that the experiment has to be terminated and a run to a special reference mark has to be made. Other mechanisms may be used to measure position. However, the environment of an electron microscope presents problems for many techniques. In particular, techniques based on magnetic fields (e.g. a linear differential transformer) may be susceptible to interference from stray magnetic fields created by an electron microscope, or may influence the performance of the microscope by its own stray field.